So I managed to remove most of the hydrangea. It was…colossal, and I ended up transplanting an offshoot in a different spot where it’s free to get as crazy-big as it wants to. What’s left is bits of the stump which I’ve painted with a bit of herbicide to make sure I’m not fighting it for the next ten years. I’ve got some catmint rooting in water and will take some cuttings from the lavender and brown-eyed susans a bit later today.
In any case, all I can say is: be careful where you plant oak-leaf hydrangeas. Very careful. They can become uncontrollable monsters.